Polymer membranes with high ionic conductivity are important for applications such as solid-state batteries and fuel cells. The performance of these materials depends not only on their electrical properties but also on other properties such as shear modulus, permeability, and the like. The mechanical properties of polymer electrolytes are particularly important in secondary solid-state lithium (Li) batteries. One of the challenges in the field of rechargeable Li ion batteries is to combine high energy density with good cyclability and electrode stability. Batteries that employ Li metal anodes for high energy density applications suffer from failures due to side reactions and dendrite growth on the Li electrodes. Repeated cycling of the batteries causes roughening of the Li surface and eventually to dendrite formation, which reduces battery life and compromises safety.
Recent theoretical work indicates that dendrite growth can be stopped if the shear modulus of current polymer electrolytes can be increased by three orders of magnitude without a significant decrease in ionic conductivity. Other studies have shown that cation transport is intimately coupled to segmental motion of the polymer chains. These studies indicate that dendrite growth on the electrode surface can be prevented by introducing a highly rigid electrolyte (elastic modulus of about 1 GPa) between the two electrodes. This high modulus requirement essentially renders most rubbery polymer electrolytes incompatible with the electrode material, as the elastic moduli of typical rubbery polymers are about 1 MPa. For example, poly(ethylene oxide) (PEO) melt, one of the most studied polymer electrolytes, has an elastic modulus of less than 1 MPa. High ionic conductivity is obtained in soft polymers such as PEO because rapid segmental motion needed for ion transport also decreases the rigidity of the polymer. Glassy polymers such as polystyrene offer very high moduli (about 3 GPa) but are poor ion conductors. Thus, conductivity and high modulus have appeared to be almost mutually exclusive goals in polymer electrolytes.
There is, therefore, a need to develop a new methodology for decoupling the electrical and mechanical properties of polymer electrolyte materials. Such a material would be useful as a solid phase electrolyte for high energy density, high cycle life batteries that do not suffer from failures due to side reactions and dendrite growth on the Li electrodes.
A separate problem encountered during operation of rechargeable lithium batteries is thermal runaway characterized by uncontrolled heating of the battery cell during operation (e.g., during charging). Such uncontrolled heating occurs because of a positive feedback which generally exists between cell temperature and conductivity of the electrolyte in a battery. As the temperature of the cell rises, the conductivity of electrolyte increases, leading to additional increase in cell temperature, leading, in turn, to an increase in electrolyte conductivity, and so on. When uncontrolled, this cycle can lead to overheating (thermal runaway) of the cell, which can cause melting of lithium metal and violent chemical reactions. Thermal runaway is a serious safety problem in lithium battery design, which is currently addressed by complex engineering solutions. Development of additional methods for preventing thermal runaway in rechargeable lithium batteries is desirable.